Silicon carbide semiconductor device and method for manufacturing the same

ABSTRACT

There is provided a silicon carbide semiconductor device having excellent electrical characteristics such as channel mobility, and a method for manufacturing the same. A semiconductor device includes a substrate made of silicon carbide and having an off-angle of greater than or equal to 50° and less than or equal to 65° with respect to a surface orientation of {0001}, a p-type layer serving as a semiconductor layer, and an oxide film serving as an insulating film. The p-type layer is formed on the substrate and is made of silicon carbide. The oxide film is formed to contact with a surface of the p-type layer. A maximum value of the concentration of nitrogen atoms in a region within 10 nm of an interface between the semiconductor layer and the insulating film (interface between a channel region and the oxide film) is greater than or equal to 1×10 21  cm −3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.13/063,083, filed Mar. 9, 2011, which is a National Stage of PCTInternational Application No. PCT/JP2009/051762, filed Feb. 3, 2009,which claims the benefit of Japanese Patent Application No. 2008-297088,filed Nov. 20, 2008, all of which are incorporated by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a silicon carbide semiconductor deviceand a method for manufacturing the same, and more particularly, to asilicon carbide semiconductor device exhibiting excellent electricalcharacteristics, and a method for manufacturing the same.

BACKGROUND ART

Conventionally, a semiconductor device using silicon carbide (SiC) hasbeen known (for example, International Publication No. WO01/018872pamphlet (that will be referred to as Patent Document 1 hereinafter)).In Patent Document 1, a SiC substrate having a surface orientation ofsubstantially {03-38} and having a 4H polytype is used to form anMOS-type field effect transistor (MOSFET) serving as a semiconductordevice. In the MOSFET, a gate oxide film is formed by dry oxidation.According to above Patent Document 1, high channel mobility (about 100cm²/Vs) can be achieved in such MOSFET.

-   Patent Document 1: International Publication No. WO01/018872    pamphlet

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As a result of the inventors' study, however, it has been found that thechannel mobility may not be sufficiently increased in some cases in theaforementioned MOSFET. In order to cause the semiconductor device usingSiC to stably exhibit the excellent characteristics, it is required toachieve high channel mobility in a reproducible manner.

The present invention has been made to solve the problems as describedabove, and an object of the present invention is to provide a siliconcarbide semiconductor device having excellent electrical characteristicssuch as channel mobility, and a method for manufacturing the same.

Means for Solving the Problems

The inventor has earnestly researched a cause of decrease in the channelmobility in order to achieve high channel mobility in a reproduciblemanner in the semiconductor device using SiC as described above, and asa result, has completed the present invention. In other words, in theabove semiconductor device, the gate oxide film is formed by the dryoxidation, and therefore, it is considered that such dry oxidation leadsto the formation of many traps (interface state) at an interface betweenthe gate oxide film and a SiC semiconductor film located under the gateoxide film. The presence of such interface state may become a factor ofthe decrease in the channel mobility described above. This is alsopresumed from the fact that a threshold voltage of the above MOSFET issignificantly high as compared with a theoretical value. Hence, theinventor has searched for a method for reducing such influence of theinterface state, and as a result, has found that the channel mobilitycan be increased by increasing the concentration of nitrogen atoms orthe concentration of hydrogen atoms in the vicinity of the aboveinterface. It is conceivable that this is because the influence of theinterface state can be suppressed by increasing the concentration of thenitrogen atoms or the concentration of the hydrogen atoms in thevicinity of the interface. Based on such finding, a silicon carbidesemiconductor device according to the present invention includes asubstrate made of silicon carbide and having an off-angle of greaterthan or equal to 50° and less than or equal to 65° with respect to asurface orientation of {0001}, a semiconductor layer and an insulatingfilm. The semiconductor layer is formed on the substrate and is made ofsilicon carbide. The insulating film is formed to contact with a surfaceof the semiconductor layer. A maximum value of a concentration ofnitrogen atoms in a region within 10 nm of an interface between thesemiconductor layer and the insulating film is greater than or equal to1×10²¹ cm⁻³.

In addition, a silicon carbide semiconductor device according to thepresent invention includes a substrate made of silicon carbide andhaving an off-angle of greater than or equal to 50° and less than orequal to 65° with respect to a surface orientation of {0001}, asemiconductor layer and an insulating film. The semiconductor layer isformed on the substrate and is made of silicon carbide. The insulatingfilm is formed to contact with a surface of the semiconductor layer. Amaximum value of a concentration of hydrogen atoms in a region within 10nm of an interface between the semiconductor layer and the insulatingfilm is greater than or equal to 1×10²¹ cm⁻³.

In addition, a silicon carbide semiconductor device according to thepresent invention includes a substrate made of silicon carbide andhaving an off-angle of greater than or equal to 50° and less than orequal to 65° with respect to a surface orientation of {0001}, asemiconductor layer and an insulating film. The semiconductor layer isformed on the substrate and is made of silicon carbide. The insulatingfilm is formed to contact with a surface of the semiconductor layer. Amaximum value of a total concentration of nitrogen atoms and hydrogenatoms in a region within 10 nm of an interface between the semiconductorlayer and the insulating film is greater than or equal to 1×10²¹ cm⁻³.

In this way, the mobility of carriers in the semiconductor layer in thevicinity of the interface between the insulating film and thesemiconductor layer (for example, the channel mobility when theinsulating film is used as the gate insulating film) can be increased ascompared with the mobility when the nitrogen atoms or the hydrogen atomsare not contained in the vicinity of the interface, and the on-stateresistance that is lower than that of a conventional semiconductordevice using silicon can be achieved. Therefore, the silicon carbidesemiconductor device with excellent electrical characteristics thatexhibits sufficiently high carrier mobility (channel mobility) can beobtained.

It is noted that the reason why the lower limit of the off-angle is setto 50° is that, as given in data that will be described hereinafter, thecarrier mobility is remarkably increased as the off-angle is increasedfrom the (01-14) surface whose off-angle is 43.3° to the (01-13) surfacewhose off-angle is 51.5°, and that there is no natural surface within arange of the off-angle of the (01-14) surface to the off-angle of the(01-13) surface described above.

In addition, the reason why the upper limit of the off-angle is set to65° is that the carrier mobility is remarkably decreased as theoff-angle is increased from the (01-12) surface whose off-angle is 62.1°to the (01-10) surface whose off-angle is 90°, and that there is nonatural surface within a range of the off-angle of the (01-12) surfaceto the off-angle of the (01-10) surface described above.

In a method for manufacturing a silicon carbide semiconductor deviceaccording to the present invention, a step of preparing a substrate madeof silicon carbide and having an off-angle of greater than or equal to50° and less than or equal to 65° with respect to a surface orientationof {0001} is first performed. A step of forming a semiconductor layer onthe substrate is performed. Furthermore, a step of forming an insulatingfilm to contact with a surface of the semiconductor layer is performed.A step of adjusting a concentration of nitrogen atoms in a region within10 nm of an interface between the semiconductor layer and the insulatingfilm such that a maximum value of the concentration of the nitrogenatoms is greater than or equal to 1×10²¹ cm⁻³ is performed.

In addition, in a method for manufacturing a silicon carbidesemiconductor device according to the present invention, a step ofpreparing a substrate made of silicon carbide and having an off-angle ofgreater than or equal to 50° and less than or equal to 65° with respectto a surface orientation of {0001} is first performed. A step of forminga semiconductor layer on the substrate is performed. Furthermore, a stepof forming an insulating film to contact with a surface of thesemiconductor layer is performed. A step of adjusting a concentration ofhydrogen atoms in a region within 10 nm of an interface between thesemiconductor layer and the insulating film such that a maximum value ofthe concentration of the hydrogen atoms is greater than or equal to1×10²¹ cm⁻³ is performed.

In addition, in a method for manufacturing a silicon carbidesemiconductor device according to the present invention, a step ofpreparing a substrate made of silicon carbide and having an off-angle ofgreater than or equal to 50° and less than or equal to 65° with respectto a surface orientation of {0001} is first performed. A step of forminga semiconductor layer on the substrate is performed. Furthermore, a stepof forming an insulating film to contact with a surface of thesemiconductor layer is performed. A step of adjusting a totalconcentration of nitrogen atoms and hydrogen atoms in a region within 10nm of an interface between the semiconductor layer and the insulatingfilm such that a maximum value of the total concentration is greaterthan or equal to 1×10²¹ cm⁻³ is performed.

In this way, the silicon carbide semiconductor device having increasedcarrier mobility (channel mobility) according to the present inventioncan be readily manufactured.

Effects of the Invention

According to the present invention, a silicon carbide semiconductordevice having high carrier mobility can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor deviceaccording to the present invention.

FIG. 2 is a flowchart for illustrating a method for manufacturing thesemiconductor device shown in FIG. 1.

FIG. 3 is a schematic cross-sectional view for illustrating each step ofthe manufacturing method shown in FIG. 2.

FIG. 4 is a schematic cross-sectional view for illustrating each step ofthe manufacturing method shown in FIG. 2.

FIG. 5 is a schematic cross-sectional view for illustrating each step ofthe manufacturing method shown in FIG. 2.

FIG. 6 is a schematic cross-sectional view for illustrating each step ofthe manufacturing method shown in FIG. 2.

FIG. 7 is a schematic cross-sectional view for illustrating each step ofthe manufacturing method shown in FIG. 2.

FIG. 8 is a schematic cross-sectional view of a second embodiment of asemiconductor device according to the present invention.

FIG. 9 is a schematic cross-sectional view for illustrating a method formanufacturing the semiconductor device shown in FIG. 8.

FIG. 10 is a schematic cross-sectional view for illustrating the methodfor manufacturing the semiconductor device shown in FIG. 8.

FIG. 11 is a schematic cross-sectional view for illustrating the methodfor manufacturing the semiconductor device shown in FIG. 8.

FIG. 12 is a schematic cross-sectional view for illustrating the methodfor manufacturing the semiconductor device shown in FIG. 8.

FIG. 13 is a schematic cross-sectional view of a third embodiment of asemiconductor device according to the present invention.

FIG. 14 is a flowchart for illustrating a method for manufacturing thesemiconductor device shown in FIG. 13.

FIG. 15 is a schematic cross-sectional view of a fourth embodiment of asemiconductor device according to the present invention.

FIG. 16 is a flowchart for illustrating a method for manufacturing thesemiconductor device shown in FIG. 15.

FIG. 17 is a flowchart for a modification of the method formanufacturing the semiconductor device shown in FIG. 16.

FIG. 18 is a graph showing the concentration of nitrogen atoms in thedepth direction of a sample in Example 1 of the present invention.

FIG. 19 is a graph showing the relationship between the measured peakvalue of the concentration of the nitrogen atoms and the channelmobility.

FIG. 20 is a graph showing the relationship between the off-angle of asubstrate in Example 2 of the present invention and the channelmobility.

FIG. 21 is a graph showing the relationship between the measured peakvalue of the total concentration of nitrogen atoms and hydrogen atomsand the channel mobility.

FIG. 22 is a schematic cross-sectional view of a semiconductor deviceprepared for measurement in Example 6.

FIG. 23 is a graph showing the CV characteristic of a sample for anexample.

FIG. 24 is a graph showing the CV characteristic of a sample for acomparative example.

FIG. 25 is a graph showing the relationship between the interface statedensity calculated from the CV characteristic shown in above FIGS. 23and 24, and the energy with respect to that of a conduction band.

FIG. 26 is a graph showing the relationship between the measured MOSchannel mobility and the interface state density.

DESCRIPTION OF THE REFERENCE SIGNS

1 semiconductor device, 2 substrate, 3 epitaxial layer, 4 p-type layer,5, 6 n⁺ region, 7, 8 oxide film, 10 gate electrode, 11 source electrode,12 drain electrode, 15 opening, 21 buffer layer, 22 voltage maintainedlayer, 23 p region, 24 n⁺ region, 25 p⁺ region, 26 oxide film, 27 uppersource electrode, 31 rear surface electrode, 41, 51 boundary region

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the drawings, wherein the same or corresponding portionsare represented by the same reference numerals, and description thereofwill not be repeated.

First Embodiment

A first embodiment of a semiconductor device according to the presentinvention will be described with reference to FIG. 1.

A semiconductor device 1 shown in FIG. 1 is a lateral MOSFET(Metal-Oxide-Semiconductor Field-Effect Transistor) serving as a siliconcarbide semiconductor device, and includes a substrate 2 made of siliconcarbide (SiC), an epitaxial layer 3 made of silicon carbide and formedon this substrate 2, a p-type layer 4 made of silicon carbide and formedon this epitaxial layer 3, n⁺ regions 5 and 6 formed in a surface ofp-type layer 4 with a spacing therebetween, an oxide film 8 serving as agate insulating film and located on a channel region between these n⁺regions 5 and 6, a gate electrode 10 formed on this oxide film 8, and asource electrode 11 and a drain electrode 12 formed on n⁺ regions 5 and6, respectively. Substrate 2 is a substrate having, as a main surface,the (03-38) surface whose off-angle is about 53° with respect to thesurface orientation of {0001}. Substrate 2 contains n-type conductiveimpurities.

Epitaxial layer 3 made of silicon carbide and formed on substrate 2 isan undoped layer. P-type layer 4 formed on this epitaxial layer 3contains conductive impurities exhibiting the p-type. In addition,conductive impurities exhibiting the n-type are implanted into n⁺regions 5 and 6. Oxide films 7 and 8 are formed to cover these p-typelayer 4 as well as n⁺ regions 5 and 6. These oxide films 7 and 8 haveopenings formed in regions located on n⁺ regions 5 and 6. Inside theopenings, source electrode 11 and drain electrode 12 electricallyconnected to n⁺ regions 5 and 6, respectively, are formed. Gateelectrode 10 is arranged on oxide film 8 acting as the gate insulatingfilm. A channel length L_(g) that refers to the distance between n⁺regions 5 and 6 can be set to, for example, approximately 100 μm. Inaddition, the channel width can be set to, for example, approximatelytwice as large as above channel length L_(g) (approximately 200 μm).

In the semiconductor device shown in FIG. 1, a maximum value of theconcentration of nitrogen atoms in a region within 10 nm of an interfacebetween p-type layer 4 serving as a semiconductor layer and oxide film 8is greater than or equal to 1×10²¹ cm⁻³. As a result, the mobility(channel mobility) in the channel region having channel length L_(g)(the region between n⁺ regions 5 and 6 in p-type layer 4) can be set toa sufficiently large value. In addition, in semiconductor device 1 shownin FIG. 1, the interface state density in an energy level lower than 0.1eV that of a conduction band is smaller than 1×10¹² cm⁻² eV⁻¹.

It is considered that the reason for this is as follows. If oxide film 8is formed by thermal oxidation and the like, the interface state isconsiderably formed at the interface between oxide film 8 and p-typelayer 4 serving as the semiconductor layer. If no measures are taken,the channel mobility in the channel region becomes extremely small ascompared with a theoretical value. In order to address this problem, thenitrogen atoms are introduced into the interface region between oxidefilm 8 and p-type layer 4 as described above, so that the influence ofthe interface state described above can be reduced and the channelmobility can be enhanced.

A method for manufacturing the first embodiment of the semiconductordevice according to the present invention will be described withreference to FIGS. 2 to 7.

First, as shown in FIG. 2, a substrate preparation step (S10) isperformed. Specifically, in this step, a silicon carbide substratehaving, as the main surface, the surface orientation of (03-38) andhaving the n-type conductivity type is prepared as substrate 2. Suchsubstrate can be obtained by using a method of cutting the substratefrom an ingot having the (0001) surface as a main surface so as toexpose the (03-38) surface as the main surface, for example.

Next, an epitaxial layer formation step (S20) is performed.Specifically, as shown in FIG. 3, undoped silicon carbide epitaxiallayer 3 is formed on substrate 2.

Next, an implantation step (S30) is performed. Specifically, theconductive impurities exhibiting the p-type conductivity (for example,aluminum (Al)) are first implanted into epitaxial layer 3 to form p-typelayer 4 as shown in FIG. 4. Next, the impurities exhibiting the n-typeconductivity type are implanted to form n⁺ regions 5 and 6 as shown inFIG. 5. Phosphorus (P), for example, can be used as these conductiveimpurities exhibiting the n-type. Any conventionally well-known methodcan be utilized to form these n⁺ regions 5 and 6. For example, the oxidefilm is formed to cover an upper surface of p-type layer 4, and then,the openings having the same two-dimensional shape patterns as those ofthe regions where n⁺ regions 5 and 6 should be formed are formed in theoxide film by photolithography and etching. Furthermore, this oxide filmhaving the patterns formed is used as a mask and the conductiveimpurities are implanted. Thus, n⁺ regions 5 and 6 described above canbe formed.

Thereafter, an activation annealing process for activating the implantedimpurities is performed. In this activation annealing process, acondition that the heating temperature is 1700° C. and the heating timeis 30 minutes may be used, for example.

Next, as shown in FIG. 2, a gate insulating film formation step (S40) isperformed. Specifically, upper surfaces of p-type layer 4 as well as n⁺regions 5 and 6 are subjected to a sacrificial oxidation process, andthen, oxide film 7 serving as the gate insulating film is formed asshown in FIG. 6. A value of, for example, 40 nm can be used as thethickness of oxide film 7.

Next, as shown in FIG. 2, a nitrogen annealing step (S50) is performed.Specifically, a nitric oxide (NO) gas is used as an atmosphere gas forheat treatment. As a condition for this heat treatment, a condition thatthe heating temperature is 1100° C. and the heating time is one hour canbe used, for example. As a result, the nitrogen atoms can be introducedinto the interface region between oxide film 7 and p-type layer 4 and n⁺regions 5, 6. In this nitrogen annealing step, an annealing step that aninert gas is used, e.g., an annealing step that an argon (Ar) gas isused as the atmosphere gas may be performed after the above annealingstep that the atmosphere gas containing the nitrogen atoms is used.

Next, as shown in FIG. 2, an electrode formation step (S60) isperformed. Specifically, a resist film having a pattern is formed onoxide film 7 by using the photolithography method. This resist film isused as a mask and oxide film 7 is partially removed to form openings 15in the regions located on n⁺ regions 5 and 6. Inside these openings 15,a conductor film that should form source electrode 11 and drainelectrode 12 as shown in FIG. 7 is formed. This conductor film is formedwith the above resist film left. Thereafter, the above resist film isremoved and the conductor film located on oxide film 7 is removed(lifted off) together with the resist film, so that source electrode 11and drain electrode 12 can be formed as shown in FIG. 7. It is notedthat oxide film 8 (a part of oxide film 7 shown in FIG. 6) locatedbetween source electrode 11 and drain electrode 12 at this time willform the gate insulating film for the semiconductor device that will beformed.

Thereafter, gate electrode 10 (see FIG. 1) is further formed on oxidefilm 8 acting as the gate insulating film. The following method can beused as a method for forming this gate electrode 10. For example, aresist film having a pattern of an opening located at a region on oxidefilm 8 is formed in advance, and a conductor film that will form thegate electrode is formed to cover the whole surface of the resist film.Then, the resist film is removed and the conductor film other than aportion of the conductor film that should form the gate electrode isremoved (lifted off). As a result, gate electrode 10 is formed as shownin FIG. 1. Thus, the semiconductor device as shown in FIG. 1 can beobtained.

Second Embodiment

A second embodiment of a semiconductor device according to the presentinvention will be described with reference to FIG. 8.

Referring to FIG. 8, a semiconductor device 1 according to the presentinvention is a vertical DiMOSFET (Double Implanted MOSFET) and includesa substrate 2, a buffer layer 21, a voltage maintained layer 22, a pregion 23, an n⁺ region 24, a p⁺ region 25, an oxide film 26, a sourceelectrode 11 and an upper source electrode 27, a gate electrode 10, anda drain electrode 12 formed on the rear surface side of substrate 2.Specifically, buffer layer 21 made of silicon carbide is formed on asurface of substrate 2 made of silicon carbide having the n conductivitytype. This buffer layer 21 has the n-type conductivity type and has athickness of, for example, 0.5 μm. In addition, the concentration ofn-type conductive impurities in the buffer layer can be set to, forexample, 5×10¹⁷ cm⁻³. Voltage maintained layer 22 is formed on thisbuffer layer 21. This voltage maintained layer 22 is made of siliconcarbide having the n-type conductivity type and has a thickness of, forexample, 10 μm. In addition, a value of 5×10¹⁵ cm⁻³ can be used as theconcentration of n-type conductive impurities in voltage maintainedlayer 22.

In a surface of this voltage maintained layer 22, p regions 23 havingthe p-type conductivity type are formed with a spacing therebetween.Within p region 23, n⁺ region 24 is formed in a surface layer of pregion 23. In addition, p⁺ region 25 is formed at the location adjacentto this n⁺ region 24. Oxide film 26 is formed to extend from a positionon n⁺ region 24 in one p region 23 through p region 23, voltagemaintained layer 22 exposed between two p regions 23, and the other pregion 23 to a position on n⁺ region 24 in the other p region 23. Gateelectrode 10 is formed on oxide film 26. In addition, source electrode11 is formed on n⁺ region 24 and p⁺ region 25. Upper source electrode 27is formed on this source electrode 11. Drain electrode 12 is formed on arear surface opposite to the surface where buffer layer 21 is formed.

A maximum value of the concentration of nitrogen atoms in a regionwithin 10 nm of an interface between oxide film 26 and n⁺ region 24, p⁺region 25, p region 23, and voltage maintained layer 22 serving as asemiconductor layer is greater than or equal to 1×10²¹ cm⁻³. In thisway, the mobility especially in a channel region under oxide film 26 (aportion of p region 23 between n⁺ region 24 and voltage maintained layer22, which comes into contact with oxide film 26) can be enhanced, as isthe case with the semiconductor device shown in FIG. 1. In addition,even in semiconductor device 1 shown in FIG. 8, the interface statedensity in an energy level lower than 0.1 eV that of a conduction bandis smaller than 1×10¹² cm⁻² eV⁻¹.

Next, a method for manufacturing the semiconductor device shown in FIG.8 will be described with reference to FIGS. 9 to 12.

A substrate preparation step (S10) is first performed similarly to themethod for manufacturing the semiconductor device shown in FIG. 2. Here,similarly to the method for manufacturing the semiconductor device inthe first embodiment of the present invention, substrate 2 (see FIG. 9)made of silicon carbide and having the (03-38) surface as a main surfaceis prepared.

In addition, a substrate having the n-type conductivity type and havinga substrate resistance of 0.02 Ωcm, for example, may be used as thissubstrate 2 (see FIG. 9).

Next, an epitaxial layer formation step (S20) is performed.Specifically, buffer layer 21 is formed on the surface of substrate 2.As the buffer layer, an epitaxial layer made of silicon carbide havingthe n-type conductivity type and having a thickness of, for example, 0.5μm is formed. A value of, for example, 5×10¹⁷ cm⁻³ can be used as theconcentration of the conductive impurities in buffer layer 21. Then,voltage maintained layer 22 is formed on this buffer layer 21 as shownin FIG. 9. A layer made of silicon carbide having the n-typeconductivity type is formed as this voltage maintained layer 22 by usingan epitaxial growth method. A value of, for example, 10 μm can be usedas the thickness of this voltage maintained layer 22. In addition, avalue of, for example, 5×10¹⁵ cm⁻³ can be used as the concentration ofthe n-type conductive impurities in this voltage maintained layer 22.

Next, an implantation step (S30) is performed similarly to the stepshown in FIG. 2. Specifically, an oxide film formed by usingphotolithography and etching is used as a mask and the impurities havingthe p-type conductivity type are implanted into voltage maintained layer22 to form p region 23 as shown in FIG. 10. The used oxide film isremoved, and then, an oxide film having a new pattern is again formed byusing photolithography and etching. Then, the oxide film is used as amask and the n-type conductive impurities are implanted into aprescribed region to form n⁺ region 24. In addition, by using thesimilar method, the conductive impurities having the p-type conductivitytype are implanted to form p⁺ region 25. As a result, the structure asshown in FIG. 10 is obtained.

After the implantation step as described above, an activation annealingprocess is performed. In this activation annealing process, the argongas can be used as the atmosphere gas, for example, and a condition thatthe heating temperature is 1700° C. and the heating time is 30 minutescan be used.

Next, a gate insulating film formation step (S40) is performed similarlyto the step shown in FIG. 2. Specifically, as shown in FIG. 11, oxidefilm 26 is formed to cover portions on voltage maintained layer 22, pregion 23, n⁺ region 24, and p⁺ region 25. As a condition for formingthis oxide film 26, dry oxidation (thermal oxidation) may be performed,for example. A condition that the heating temperature is 1200° C. andthe heating time is 30 minutes can be used as a condition for this dryoxidation.

Thereafter, a nitrogen annealing step (S50) is performed similarly tothe step shown in FIG. 2. Specifically, nitric oxide (NO) is used as theatmosphere gas to perform the annealing process. As a temperaturecondition for the annealing process, the heating temperature is set to1100° C. and the heating time is set to 120 minutes, for example. As aresult, the nitrogen atoms are introduced in the vicinity of theinterface between oxide film 26 and voltage maintained layer 22, pregion 23, n⁺ region 24, and p⁺ region 25 that are lower layers. Inaddition, after this annealing step that the nitric oxide is used as theatmosphere gas, additional annealing may be performed by using the argon(Ar) gas that is the inert gas. Specifically, the argon gas may be usedas the atmosphere gas, and a condition that the heating temperature is1100° C. and the heating time is 60 minutes may be used.

Next, an electrode formation step (S60) is performed similarly to thestep shown in FIG. 2. Specifically, a resist film having a pattern isformed on oxide film 26 by using the photolithography method. The resistfilm is used as a mask and a portion of the oxide film located on n⁺region 24 and p⁺ region 25 is removed by etching. Thereafter, aconductor film such as metal is formed on the resist film and is formedinside the opening formed in oxide film 26 to contact with n⁺ region 24and p⁺ region 25. Thereafter, the resist film is removed and theconductor film located on the resist film is removed (lifted off). Here,nickel (Ni), for example, can be used as a conductor. As a result, asshown in FIG. 12, source electrode 11 and drain electrode 12 can beobtained. It is noted that heat treatment for alloying is preferablyperformed at this time. Specifically, the argon (Ar) gas that is theinert gas is used as the atmosphere gas, for example, and the heattreatment (alloying process) is performed under the condition that theheating temperature is 950° C. and the heating time is 2 minutes.

Thereafter, upper source electrode 27 (see FIG. 8) is formed on sourceelectrode 11. In addition, drain electrode 12 (see FIG. 8) is formed onthe rear surface of substrate 2. Thus, the semiconductor device shown inFIG. 8 can be obtained.

Third Embodiment

A third embodiment of a semiconductor device according to the presentinvention will be described with reference to FIG. 13.

Referring to FIG. 13, a semiconductor device 1 according to the presentinvention basically has a configuration similar to that of semiconductordevice 1 shown in FIG. 1. Semiconductor device 1 according to thepresent invention, however, differs from semiconductor device 1 shown inFIG. 1 in that a maximum value of the concentration of hydrogen atoms ina boundary region 41 including a region within 10 nm of an interfacebetween a p-type layer 4 serving as a semiconductor layer and an oxidefilm 8 is greater than or equal to 1×10²¹ cm⁻³. Even in such aconfiguration, the mobility (channel mobility) in a channel regionincluding boundary region 41 can be set to a sufficiently large value,as is the case with the semiconductor device shown in FIG. 1. It isconsidered that this is because the hydrogen atoms contained in boundaryregion 41 reduce the interface state in semiconductor device 1 shown inFIG. 13, similarly to the nitrogen atoms contained in the region within10 nm of the interface between p-type layer 4 and oxide film 8 ofsemiconductor device 1 shown in FIG. 1. In other words, even in thesemiconductor device shown in FIG. 13, the interface state density in anenergy level lower than 0.1 eV that of a conduction band is smaller than1×10¹² cm⁻² eV⁻¹.

A method for manufacturing the third embodiment of the semiconductordevice according to the present invention will be described withreference to FIG. 14.

The method for manufacturing the semiconductor device shown in FIG. 14is basically similar to the method for manufacturing the semiconductordevice shown in FIG. 2. The method for manufacturing the semiconductordevice shown in FIG. 14, however, differs from the method formanufacturing the semiconductor device shown in FIG. 2 in that ahydrogen annealing step (S70) is performed instead of the nitrogenannealing step (S50) in FIG. 2. Specifically, a substrate preparationstep (S10), an epitaxial layer formation step (S20), an implantationstep (S30), and a gate insulating film formation step (S40) areperformed similarly to the manufacturing method shown in FIG. 2.Thereafter, the hydrogen annealing step (S70) is performed.Specifically, a hydrogen gas (H₂) is used as the atmosphere gas for heattreatment. As a condition for this heat treatment, a condition that theheating temperature is 1100° C. and the heating time is one hour can beused, for example. As a result, the hydrogen atoms can be introducedinto an interface region between an oxide film 7 and p-type layer 4 andn⁺ regions 5, 6. In addition, in this hydrogen annealing step, anannealing step that the inert gas is used, e.g., an annealing step thatthe argon (Ar) gas is used as the atmosphere gas may be performed afterthe above annealing step that the atmosphere gas containing the hydrogenatoms is used. Moreover, in the above hydrogen annealing step (S70),water vapor or water vapor-containing hydrogen gas may be used as theatmosphere gas instead of the hydrogen gas.

Thereafter, as shown in FIG. 14, an electrode formation step (S60) isperformed similarly to the manufacturing method shown in FIG. 2, andthus, semiconductor device 1 shown in FIG. 13 can be obtained.

Fourth Embodiment

A fourth embodiment of a semiconductor device according to the presentinvention will be described with reference to FIG. 15.

Referring to FIG. 15, a semiconductor device 1 according to the presentinvention basically has a configuration similar to that of semiconductordevice 1 shown in FIG. 1. Semiconductor device 1 according to thepresent invention, however, differs from semiconductor device 1 shown inFIG. 1 in that a maximum value of the total concentration of nitrogenatoms and hydrogen atoms in a boundary region 51 including a regionwithin 10 nm of an interface between a p-type layer 4 serving as asemiconductor layer and an oxide film 8 is greater than or equal to1×10²¹ cm⁻³. Even in such a configuration, the mobility (channelmobility) in a channel region including boundary region 51 can be set toa sufficiently large value, as is the case with the semiconductor deviceshown in FIG. 1. In addition, even in the semiconductor device shown inFIG. 15, the interface state density in an energy level lower than 0.1eV that of a conduction band is smaller than 1×10¹² cm⁻² eV⁻¹.

A method for manufacturing the fourth embodiment of the semiconductordevice according to the present invention will be described withreference to FIG. 16.

The method for manufacturing the semiconductor device shown in FIG. 16is basically similar to the method for manufacturing the semiconductordevice shown in FIG. 2. The method for manufacturing the semiconductordevice shown in FIG. 16, however, differs from the method formanufacturing the semiconductor device shown in FIG. 2 in that ahydrogen annealing step (S70) is performed after a nitrogen annealingstep (S50) and before an electrode formation step (S60) in FIG. 16.Specifically, a substrate preparation step (S10), an epitaxial layerformation step (S20), an implantation step (S30), a gate insulating filmformation step (S40), and the nitrogen annealing step (S50) areperformed similarly to the manufacturing method shown in FIG. 2.Thereafter, the hydrogen annealing step (S70) is performed. In this step(S70), a condition similar to that for the hydrogen annealing step (S70)in the manufacturing method in the third embodiment (the condition forannealing where the hydrogen gas is used) can be used. As a result, thenitrogen atoms and the hydrogen atoms can be introduced into aninterface region between an oxide film 7 and p-type layer 4 and n⁺regions 5, 6. It is noted that, in the above hydrogen annealing step(S70), water vapor or water vapor-containing oxygen gas may be used asthe atmosphere gas instead of the hydrogen gas. In addition, thehydrogen annealing step (S70) may be performed before the nitrogenannealing step (S50). Moreover, the hydrogen annealing step (S70) andthe nitrogen annealing step (S50) may be simultaneously performed byheat treatment that the atmosphere gas containing the hydrogen atoms andthe nitrogen atoms is used.

Thereafter, as shown in FIG. 16, the electrode formation step (S60) isperformed similarly to the manufacturing method shown in FIG. 2, andthus, semiconductor device 1 shown in FIG. 15 can be obtained.

A modification of the method for manufacturing the fourth embodiment ofthe semiconductor device according to the present invention will bedescribed with reference to FIG. 17.

A method for manufacturing a semiconductor device shown in FIG. 17 isbasically similar to the method for manufacturing the semiconductordevice shown in FIG. 16. The method for manufacturing the semiconductordevice shown in FIG. 17, however, differs from the method formanufacturing the semiconductor device shown in FIG. 16 in that a postheat treatment step (S80) is performed after the hydrogen annealing step(S70) and before the electrode formation step (S60) in FIG. 16.Specifically, a substrate preparation step (S10), an epitaxial layerformation step (S20), an implantation step (S30), a gate insulating filmformation step (S40), a nitrogen annealing step (S50), and a hydrogenannealing step (S70) are performed similarly to the manufacturing methodshown in FIG. 16. Thereafter, the post heat treatment step (S80) isperformed. Specifically, an annealing step that the inert gas is used isperformed. As a condition for this annealing step, the inert gas (forexample, argon (Ar)) can be used as the atmosphere gas, and a conditionthat the heating temperature is 1100° C. and the heating time is 60minutes can be used. Such annealing step that the inert gas is usedallows more reliable production of an effect of reducing the interfacestate by the nitrogen atoms and the hydrogen atoms introduced into achannel region through the nitrogen annealing step (S50) and thehydrogen annealing step (S70).

Thereafter, as shown in FIG. 17, an electrode formation step (S60) isperformed similarly to the manufacturing method shown in FIG. 2, andthus, semiconductor device 1 shown in FIG. 15 can be obtained.

It is noted that a heat treatment step similar to the above post heattreatment step (S80) may be additionally performed between the nitrogenannealing step (S50) and the hydrogen annealing step (S70). In addition,in addition, in the manufacturing method shown in FIG. 17, the hydrogenannealing step (S70) may also be performed before the nitrogen annealingstep (S50). Moreover, the hydrogen annealing step (S70) and the nitrogenannealing step (S50) may be simultaneously performed by heat treatmentthat the atmosphere gas containing the hydrogen atoms and the nitrogenatoms is used.

Although the lateral MOSFET is shown as semiconductor device 1 in thethird and fourth embodiments described above, the characteristics of thethird and fourth embodiments may be applied to the vertical DiMOSFETshown in FIG. 8. In other words, in semiconductor device 1 shown in FIG.8, the maximum value of the concentration of the hydrogen atoms in theregion within 10 nm of the interface between oxide film 26 and n⁺ region24, p⁺ region 25, p region 23, and voltage maintained layer 22 servingas the semiconductor layer, or the maximum value of the totalconcentration of the nitrogen atoms and the hydrogen atoms can be set togreater than or equal to 1×10²¹ cm⁻³.

In addition, it is preferable that substrate 2 forming semiconductordevice 1 shown in the above first to fourth embodiments has anoff-orientation in a range of less than or equal to ±5° in the <11-20>direction or in a range of less than or equal to ±5° in the <01-10>direction. Moreover, it is more preferable that the main surface ofsubstrate 2 forming semiconductor device 1 in the above first to fourthembodiments has a surface orientation whose off-angle is greater than orequal to −3° and less than or equal to +5° with respect to the surfaceorientation of {03-38}.

Here, the characteristic configuration of the present invention will beenumerated, a part of which overlaps those in the above embodiments.

A semiconductor device 1 serving as a silicon carbide semiconductordevice according to the present invention includes a substrate 2 made ofsilicon carbide and having an off-angle of greater than or equal to 50°and less than or equal to 65° with respect to a surface orientation of{0001}, a semiconductor layer (p-type layer 4 in FIG. 1, p region 23 inFIG. 8), and an insulating film (oxide film 8 in FIG. 1, oxide film 26in FIG. 8). The semiconductor layer (p-type layer 4, p region 23) isformed on substrate 2 and is made of silicon carbide. The insulatingfilm (oxide film 8, 26) is formed to contact with a surface of thesemiconductor layer (p-type layer 4 including the channel region, pregion 23). A maximum value of a concentration of nitrogen atoms in aregion within 10 nm of an interface between the semiconductor layer andthe insulating film (interface between the channel region and oxide film8, 26) is greater than or equal to 1×10²¹ cm³.

In this way, the mobility of carriers (channel mobility) in a channelregion in the vicinity of the interface between oxide film 8, 26 actingas a gate insulating film and the channel region can be increased ascompared with a case where the nitrogen atoms are not contained in thevicinity of the interface, and the on-state resistance that is lowerthan that of the conventional semiconductor device using silicon can beachieved. Therefore, semiconductor device 1 with excellent electricalcharacteristics that exhibits sufficiently high channel mobility can beobtained. It is noted that the reason why the maximum value of theconcentration of the nitrogen atoms is set to greater than or equal to1×10²¹ cm⁻³ as described above is that, when the concentration of thenitrogen atoms is set to greater than or equal to the above value, thechannel mobility can be set to a practically sufficient value, that is,greater than or equal to 50 cm²/Vs.

In above semiconductor device 1, hydrogen atoms may be contained in theregion within 10 nm of the interface between the semiconductor layer(p-type layer 4 in FIG. 1, p region 23 in FIG. 8) and the insulatingfilm (oxide film 8, 26). In this case, the interface state in the regioncan be reduced more reliably.

A semiconductor device 1 serving as a silicon carbide semiconductordevice according to the present invention includes a substrate 2 made ofsilicon carbide and having an off-angle of greater than or equal to 50°and less than or equal to 65° with respect to a surface orientation of{0001}, a semiconductor layer (p-type layer 4 in FIG. 13, p region 23 inFIG. 8), and an insulating film (oxide film 8 in FIG. 13, oxide film 26in FIG. 8). The semiconductor layer (p-type layer 4, p region 23) isformed on substrate 2 and is made of silicon carbide. The insulatingfilm (oxide film 8, 26) is formed to contact with a surface of thesemiconductor layer (p-type layer 4 including the channel region, pregion 23). A maximum value of a concentration of hydrogen atoms in aregion within 10 nm of an interface between the semiconductor layer andthe insulating film (for example, an interface between the channelregion and oxide film 8, 26 included in boundary region 41 in FIG. 13)is greater than or equal to 1×10²¹ cm⁻³.

In this way, the mobility of carriers in a channel region in thevicinity of the interface between oxide film 8, 26 acting as a gateinsulating film and the channel region can be increased as compared witha case where the hydrogen atoms are not contained in the vicinity of theinterface, and the on-state resistance that is lower than that of theconventional semiconductor device using silicon can be achieved. It isnoted that the reason why the maximum value of the concentration of thehydrogen atoms is set to greater than or equal to 1×10²¹ cm⁻³ asdescribed above is that, when the concentration of the hydrogen atoms isset to greater than or equal to the above value, the channel mobilitycan be set to a practically sufficient value, that is, greater than orequal to 50 cm²/Vs.

In above semiconductor device 1, nitrogen atoms may be contained in theregion within 10 nm of the interface between the semiconductor layer(p-type layer 4 in FIG. 13, p region 23 in FIG. 8) and the insulatingfilm (oxide film 8 in FIG. 13, oxide film 26 in FIG. 8). In this case,the interface state in the region can be reduced more reliably.

A semiconductor device 1 serving as a silicon carbide semiconductordevice according to the present invention includes a substrate 2 made ofsilicon carbide and having an off-angle of greater than or equal to 50°and less than or equal to 65° with respect to a surface orientation of{0001}, a semiconductor layer (p-type layer 4 in FIG. 15, p region 23 inFIG. 8), and an insulating film (oxide film 8 in FIG. 15, oxide film 26in FIG. 8). The semiconductor layer (p-type layer 4, p region 23) isformed on substrate 2 and is made of silicon carbide. The insulatingfilm (oxide film 8, 26) is formed to contact with a surface of thesemiconductor layer (p-type layer 4 including the channel region, pregion 23). A maximum value of a total concentration of nitrogen atomsand hydrogen atoms in a region within 10 nm of an interface between thesemiconductor layer and the insulating film (for example, an interfacebetween the channel region and oxide film 8, 26 included in boundaryregion 51 in FIG. 15) is greater than or equal to 1×10²¹ cm⁻³.

In this way, the mobility of carriers in the channel region in thevicinity of the interface between oxide film 8, 26 acting as a gateinsulating film and the channel region can be increased as compared witha case where the nitrogen atoms and the hydrogen atoms are not containedin the vicinity of the interface, and the on-state resistance that islower than that of the conventional semiconductor device using siliconcan be achieved. It is noted that the reason why the maximum value ofthe total concentration of the nitrogen atoms and the hydrogen atoms isset to greater than or equal to 1×10²¹ cm⁻³ as described above is that,when the total concentration is set to greater than or equal to theabove value, the channel mobility can be set to a practically sufficientvalue, that is, greater than or equal to 50 cm²/Vs.

In above semiconductor device 1, an interface state density in an energylevel lower than 0.1 eV that of a conduction band is preferably smallerthan 1×10¹² cm⁻² eV⁻¹. In this case, the interface state density asdescribed above allows the mobility of the carriers in the channelregion to be sufficiently increased. It is noted that, when the aboveinterface state density is larger than 1×10¹² cm⁻² eV⁻¹, the channelmobility in semiconductor device 1 falls below 50 cm²/Vs that isconsidered as the practically sufficient value, and therefore, it ispreferable to set a value of the interface state density to smaller than1×10¹² cm⁻² eV⁻¹ as described above.

In above semiconductor device 1, substrate 2 may have an off-orientationin a range of less than or equal to ±5° in a <11-20> direction. Inaddition, substrate 2 made of silicon carbide may be a SiC substratehaving a 4H polytype. In above semiconductor device 1, substrate 2 mayhave an off-orientation in a range of less than or equal to ±5° in a<01-10> direction. In this case, the above off-orientation is a typicaloff-orientation in the SiC substrate having the 4H polytype, and anepitaxial layer can be readily formed on the SiC substrate. It is notedthat, in consideration of processing variations during slicing of thesubstrate, the range of the off-orientation is set to less than or equalto ±5°, respectively.

In above semiconductor device 1, a main surface of substrate 2 may havea surface orientation whose off-angle is greater than or equal to −3°and less than or equal to +5° with respect to a surface orientation of{03-38}. In addition, more preferably, the main surface of the substratehas the surface orientation of substantially {03-38}, and still morepreferably, the main surface of the substrate has the surfaceorientation of {03-38}. Here, a state in which the main surface of thesubstrate has the surface orientation of substantially {03-38} meansthat, because of the processing accuracy and the like of the substrate,the surface orientation of the main surface of the substrate is withinthe range of the off-angle where the surface orientation can be regardedas substantially {03-38}. The range of the off-angle in this case issuch that the off-angle is ±2° with respect to {03-38}, for example. Inthis case, the carrier mobility (channel mobility) described above canbe maximized.

It is noted that the reason why the range of the off-angle in anarbitrary direction with respect to the surface orientation of {03-38}is set to greater than or equal to −3° and less than or equal to +5° isthat, as can be seen from data that will be described hereinafter, atleast the above range can be considered as the range of the off-anglewhere the channel mobility of greater than or equal to approximately 90cm²/Vs that is considered as the excellent carrier mobility (channelmobility) is obtained.

In a method for manufacturing a silicon carbide semiconductor deviceaccording to the present invention, a step of preparing a substrate 2made of silicon carbide and having an off-angle of greater than or equalto 50° and less than or equal to 65° with respect to a surfaceorientation of {0001} (substrate preparation step (S10)) is firstperformed. A step of forming a semiconductor layer on substrate 2(epitaxial layer formation step (S20)) is performed. Furthermore, a stepof forming an insulating film to contact with a surface of thesemiconductor layer (gate insulating film formation step (S40)) isperformed. A step of adjusting a concentration of nitrogen atoms in aregion within 10 nm of an interface between the semiconductor layer andthe insulating film such that a maximum value of the concentration ofthe nitrogen atoms is greater than or equal to 1×10²¹ cm⁻³ (nitrogenannealing step (S50)) is performed. In this way, a semiconductor device1 according to the present invention having increased carrier mobility(channel mobility) can be readily manufactured.

The above method for manufacturing a silicon carbide semiconductordevice may further include a step of introducing hydrogen atoms into theregion within 10 nm of the interface between the semiconductor layer(p-type layer 4, p region 23) and the insulating film (oxide film 8, 26)(for example, hydrogen annealing step (S70) in FIG. 16 or 17). In thiscase, the silicon carbide semiconductor device containing the hydrogenatoms in addition to the nitrogen atoms can be readily manufactured inthe above region.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of introducing hydrogen atoms (hydrogen annealing step(S70)) includes a step of using a gas containing the hydrogen atoms asan atmosphere gas for heat treatment of the substrate where theinsulating film (oxide film 8, 26) is formed. In this case, theconcentration of the hydrogen atoms in the vicinity of the interfacebetween the semiconductor layer (p-type layer 4 including the channelregion, p region 23) and oxide film 8, 26 can be readily adjusted.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of introducing hydrogen atoms (hydrogen annealing step(S70)) may include a step of using the gas containing the hydrogen atomsas the atmosphere gas for heat treatment, and then, using an inert gasas the atmosphere gas for heat treatment of the substrate. In this case,the carrier mobility in semiconductor device 1 can be further increased.

In the above method for manufacturing a silicon carbide semiconductordevice, the nitrogen annealing step (S50) may include a step of using agas containing the nitrogen atoms as an atmosphere gas for heattreatment of substrate 2 where the insulating film (oxide film 8, 26) isformed. In this case, the concentration of the nitrogen atoms in thevicinity of the interface between the semiconductor layer (p-type layer4 including the channel region, p region 23) and oxide film 8, 26 can bereadily adjusted.

In the above method for manufacturing a silicon carbide semiconductordevice, the nitrogen annealing step (S50) may include a step of usingthe above gas containing the nitrogen atoms as the atmosphere gas forheat treatment, and then, using an inert gas (Ar gas) as the atmospheregas for heat treatment of substrate 2. In this case, the carriermobility in semiconductor device 1 can be further increased.

In a method for manufacturing a silicon carbide semiconductor deviceaccording to the present invention, a step of preparing a substrate 2made of silicon carbide and having an off-angle of greater than or equalto 50° and less than or equal to 65° with respect to a surfaceorientation of {0001} (substrate preparation step (S10)) is firstperformed. A step of forming a semiconductor layer on substrate 2(epitaxial layer formation step (S20)) is performed. Furthermore, a stepof forming an insulating film to contact with a surface of thesemiconductor layer (gate insulating film formation step (S40)) isperformed. A step of adjusting a concentration of hydrogen atoms in aregion within 10 nm of an interface between the semiconductor layer andthe insulating film such that a maximum value of the concentration ofthe hydrogen atoms is greater than or equal to 1×10²¹ cm⁻³ (hydrogenannealing step (S70)) is performed. In this way, a semiconductor device1 according to the present invention having increased carrier mobility(channel mobility) can be readily manufactured.

The above method for manufacturing a silicon carbide semiconductordevice may further include a step of introducing nitrogen atoms into theregion within 10 nm of the interface between the semiconductor layer(p-type layer 4, p region 23) and the insulating film (oxide film 8, 26)(nitrogen annealing step (S50)). In this case, the silicon carbidesemiconductor device containing the nitrogen atoms in addition to thehydrogen atoms can be readily manufactured in the above region.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of introducing nitrogen atoms (nitrogen annealing step(S50)) includes a step of using a gas containing the nitrogen atoms asan atmosphere gas for heat treatment of the substrate where theinsulating film (oxide film 8, 26) is formed. In this case, theconcentration of the nitrogen atoms in the vicinity of the interfacebetween the semiconductor layer (p-type layer 4 including the channelregion, p region 23) and oxide film 8, 26 can be readily adjusted.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of introducing nitrogen atoms (nitrogen annealing step(S50)) may include a step of using the gas containing the nitrogen atomsas the atmosphere gas for heat treatment, and then, using an inert gasas the atmosphere gas for heat treatment of the above-describedsubstrate. In this case, the carrier mobility in semiconductor device 1can be further increased.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of adjusting a concentration of hydrogen atoms(hydrogen annealing step (S70)) may include a step of using a gascontaining the hydrogen atoms as an atmosphere gas for heat treatment ofthe substrate where the insulating film (oxide film 8, 26) is formed. Inthis case, the concentration of the hydrogen atoms in the vicinity ofthe interface between the semiconductor layer (p-type layer 4 includingthe channel region, p region 23) and oxide film 8, 26 can be readilyadjusted.

In the above method for manufacturing a silicon carbide semiconductordevice, the step of adjusting a concentration of hydrogen atoms(hydrogen annealing step (S70)) may include a step of using the gascontaining the hydrogen atoms as the atmosphere gas for heat treatment,and then, using an inert gas as the atmosphere gas for heat treatment ofthe substrate. In this case, the carrier mobility in semiconductordevice 1 can be further increased.

In the above method for manufacturing a silicon carbide semiconductordevice, the gas containing the hydrogen atoms may be water vapor orwater vapor-containing oxygen gas. In this case, since the water vaporthat is easily available and easy to handle is used, the above hydrogenannealing step (S70) can be performed with relative ease.

In a method for manufacturing a silicon carbide semiconductor deviceaccording to the present invention, a step of preparing a substrate 2made of silicon carbide and having an off-angle of greater than or equalto 50° and less than or equal to 65° with respect to a surfaceorientation of {0001} (substrate preparation step (S10)) is performed. Astep of forming a semiconductor layer on substrate 2 (epitaxial layerformation step (S20)) is performed. Furthermore, a step of forming aninsulating film to contact with a surface of the semiconductor layer(gate insulating film formation step (S40)) is performed. A step ofadjusting a total concentration of nitrogen atoms and hydrogen atoms ina region within 10 nm of an interface between the semiconductor layerand the insulating film such that a maximum value of the totalconcentration is greater than or equal to 1×10²¹ cm⁻³ (nitrogenannealing step (S50) and hydrogen annealing step (S70)) is performed. Inthis way, a semiconductor device 1 according to the present inventionhaving increased carrier mobility (channel mobility) can be readilymanufactured.

Example 1

The details of an experiment conducted to check the effect of thepresent invention will be described hereinafter.

(As to Sample)

A semiconductor device having a structure shown in FIG. 1 was fabricatedas a sample as follows. An epitaxial layer 3 having a thickness of 10 μmwas formed on an n-type silicon carbide substrate 2 having a thicknessof 400 μm, and a p-type layer 4 having a thickness of 1 μm was formed onepitaxial layer 3. Then, phosphorus (P) was implanted as n-typeconductive impurities for n⁺ regions 5 and 6, when a value of 1×10²°cm⁻³ was used as the concentration of the impurities. In addition, thegate length (channel length L_(g)) that refers to the distance betweenn⁺ regions 5 and 6 was set to 100 μm. Moreover, the gate width (channelwidth) was set to 200 μm.

A sample that was subjected to nitrogen annealing after formation of anoxide film by a dry oxidation process was fabricated as a sample forExample 1 of the present invention. In addition, a sample that wassubjected to the nitrogen annealing after formation of an oxide film,and further subjected to an annealing process where the argon gasserving as the inert gas was used as the atmosphere (argon annealingprocess) was fabricated as a sample for Example 2 of the presentinvention. Here, as a condition for the dry oxidation process to form anoxide film 8, a condition that the heating temperature was 1200° C. andthe heating time was 30 minutes was used. In the nitrogen annealing stepfor the sample for Example 1 of the present invention, the nitric oxidegas was used as the atmosphere gas, and the heating temperature was setto 1100° C. and the heating time was set to 60 minutes. In the nitrogenannealing step for the sample for Example 2 of the present invention,the nitric oxide gas was used as the atmosphere gas, and the heatingtemperature was set to 1100° C. and the heating time was set to 120minutes. In the argon annealing process for the sample for Example 2 ofthe present invention, the argon gas was used as the atmosphere gas, anda condition that the heating temperature was 1100° C. and the heatingtime was 60 minutes was used.

In addition, a sample that was not subjected to the nitrogen annealingstep after formation of a gate insulating film was fabricated as asample for a comparative example. The above oxide film in Example 1 hada thickness of 40 nm, the oxide film in Example 2 had a thickness of 46nm, and the oxide film in the comparative example had a thickness of 33nm.

After the insulating film as described above was formed, a sourceelectrode 11 and a drain electrode 12 were formed, and further, a gateelectrode 10 was formed on oxide film 8 serving as the gate insulatingfilm as shown in FIG. 1, similarly to the manufacturing method in thefirst embodiment of the present invention. Source electrode 11 and drainelectrode 12 were made of nickel (Ni) and had a thickness of 0.1 μm. Inaddition, gate electrode 10 was made of aluminum (Al) and had athickness of 1 μm.

(Method of Measurement)

The concentration distribution of nitrogen atoms in the depth directionin the vicinity of an interface between oxide film 8 and p-type layer 4serving as a semiconductor layer was measured as to each of the samplesdescribed above. The SIMS (secondary ion mass spectroscopy) was used asa method of measurement. In addition, the channel mobility was measuredin the formed semiconductor devices. The following method was used as amethod of measurement. A source-drain voltage was set to V_(DS)=0.1V anda gate voltage VG was applied to measure a source-drain current I_(DS)(to measure the gate voltage dependency). Then, assuming thatg_(m)=(δI_(DS))/(δV_(G)), a maximum value of the channel mobility withrespect to the gate voltage was determined from the following equation:

channel mobility μ=g _(m)×(L×d)/(W×ε×V _(DS))

(where L: gate length, d: thickness of the oxide film, W: gate width, ε:permittivity of the oxide film).

(Result of Measurement)

The concentration distribution of the nitrogen atoms in the depthdirection was basically as shown in FIG. 18. In FIG. 18, the horizontaldirection indicates the depth from the surface of the oxide film and itis expressed in the unit nm. The vertical axis indicates theconcentration of the nitrogen atoms (it is expressed in the unit cm⁻³).As can be seen from FIG. 18, the nitrogen atoms had the highestconcentration in the interface portion between oxide film 8 and p-typelayer 4 serving as the semiconductor layer. It is seen that the nitrogenatoms were distributed within a range of ±10 nm around the interfacebetween oxide film 8 and p-type layer 4. Although FIG. 18 shows themeasurement data about Example 1, the substantially similarconcentration distribution of the nitrogen atoms was also obtained as toExample 2. In Example 2, however, the maximum value (peak value) of theconcentration of the nitrogen atoms was higher than that in Example 1.

Next, the result of the measurement of the mobility in the channel isshown in FIG. 19. The horizontal axis in FIG. 19 indicates the peakvalue of the concentration of the nitrogen atoms (peak concentration ofthe nitrogen atoms) measured in each sample. It is expressed in the unitcm⁻³. The vertical axis in FIG. 19 indicates the channel mobility in themeasured semiconductor devices (MOS channel mobility). It is expressedin the unit cm²/Vs.

As shown in FIG. 19, the sample for the comparative example had thelowest peak concentration of the nitrogen atoms, and at the same time,had the lowest value of the channel mobility. On the other hand, thesamples for Examples 1 and 2 both had higher peak concentration of thenitrogen atoms, and at the same time, had larger values of the channelmobility than that of the sample for the comparative example. Comparingthe sample for Example 1 and the sample for Example 2, the sample forExample 2 having higher peak concentration of the nitrogen atoms had alarger value of the channel mobility than that of the sample for Example1.

Here, as compared with the conventional MOSFET using silicon, a minimumvalue required for the channel mobility to achieve lower on-stateresistance is considered as 50 cm²/Vs. Therefore, even in considerationof variations in the process, it is considered that a sufficient valueof the channel mobility can be achieved if the peak concentration of thenitrogen atoms is set to greater than or equal to 1×10²¹ cm⁻³, based onFIG. 19.

Example 2

Next, the relationship between the off-angle of substrate 2 and thechannel mobility was checked. The specific description will follow.

(Sample)

A sample was fabricated by using a manufacturing method similar to themethod for manufacturing the sample for Example 2 described above.Specifically, different surface orientations of main surfaces ofsubstrates were used to fabricate four types of samples for comparativeexamples and three types of samples for examples of the presentinvention. In other words, a silicon carbide substrate whose mainsurface has a surface orientation where an off-angle is 8° with respectto (0001) (8°-off substrate with respect to (0001)) was prepared as acomparative example 1. A substrate whose main surface has a surfaceorientation indicated by (01-15) was prepared as a comparative example2. A substrate whose main surface has a surface orientation indicated by(01-14) was prepared as a comparative example 3. A substrate whose mainsurface has a surface orientation where an off-angle is 70° with respectto (0001) was prepared as a comparative example 4. In addition, as forthe examples of the present invention, a substrate whose main surfacehas a surface orientation indicated by (01-13) was prepared asExample 1. A substrate whose main surface has a surface orientationindicated by (03-38) was prepared as Example 2. A substrate whose mainsurface has a surface orientation indicated by (01-12) was prepared asExample 3. Then, these different substrates were used to formsemiconductor devices having similar structures as the samples describedabove.

(Method of Measurement)

The channel mobility was measured as to each of the samples describedabove. As a method for measuring the channel mobility, a methodbasically similar to the method for measuring the channel mobility inExample 1 was used.

(Result of Measurement)

The result of the measurement is shown in FIG. 20. The horizontal axisin FIG. 20 indicates the off-angle (unit: °) of the main surface of thesubstrate forming each sample with respect to the surface orientation of{0001}. The vertical axis indicates the channel mobility (unit: cm²/Vs)similarly to the vertical axis in FIG. 19. As can be seen from FIG. 20,in the samples for Examples 1 to 3 having the off-angles in the rangecorresponding to the examples of the present invention (greater than orequal to 50° and less than or equal to 65°), the values of the channelmobility were significantly increased as compared with those of thecomparative examples.

Example 3

Next, the details of an experiment conducted to check the effect whenhydrogen atoms are introduced into a region within 10 nm of an interfacebetween a semiconductor layer and an insulating film will be described.

(As to Sample)

A semiconductor device having a structure shown in FIG. 1 was fabricatedas a sample as follows. An epitaxial layer 3 having a thickness of 10 μmwas formed on an n-type silicon carbide substrate 2 having a thicknessof 400 μm, and a p-type layer 4 having a thickness of 1 μm was formed onepitaxial layer 3. Then, phosphorus (P) was implanted as n-typeconductive impurities for n⁺ regions 5 and 6, when a value of 1×10²⁰cm⁻³ was used as the concentration of the impurities. In addition, thegate length (channel length L_(g)) that refers to the distance betweenn⁺ regions 5 and 6 was set to 100 μm. Moreover, the gate width (channelwidth) was set to 200 μm.

A sample that was subjected to hydrogen annealing after formation of anoxide film by a dry oxidation process was fabricated as a sample forExample 1 of the present invention. In addition, a sample that wassubjected to the hydrogen annealing after formation of an oxide film,and further subjected to an annealing process where the argon gasserving as the inert gas was used as the atmosphere (argon annealingprocess) was fabricated as a sample for Example 2 of the presentinvention. Here, as a condition for the dry oxidation process to form anoxide film 8, a condition that the heating temperature was 1200° C. andthe heating time was 30 minutes was used. In the hydrogen annealing stepfor the sample for Example 1 of the present invention, the hydrogen gaswas used as the atmosphere gas, and the heating temperature was set to1100° C. and the heating time was set to 60 minutes. As a condition forthe hydrogen annealing for the sample for Example 2 of the presentinvention, the hydrogen gas was used as the atmosphere gas, and acondition that the heating temperature was 1100° C. and the heating timewas 120 minutes was used. In the argon annealing process for the samplefor Example 2, the argon gas was used as the atmosphere gas, and theheating temperature was set to 1100° C. and the heating time was set to60 minutes.

In addition, a sample that was not subjected to the hydrogen annealingstep after formation of a gate insulating film was fabricated as asample for a comparative example. It is noted that the above oxide filmin Example 1 had a thickness of 40 nm, the oxide film in Example 2 had athickness of 45 nm, and the oxide film in the comparative example had athickness of 33 nm.

After the insulating film as described above was formed, a sourceelectrode 11 and a drain electrode 12 were formed, and further, a gateelectrode 10 was formed on oxide film 8 serving as the gate insulatingfilm as shown in FIG. 1, similarly to the manufacturing method in thefirst embodiment of the present invention. Source electrode 11 and drainelectrode 12 were made of nickel (Ni) and had a thickness of 0.1 μm. Inaddition, gate electrode 10 was made of aluminum (Al) and had athickness of 1 μm.

(Method of Measurement)

The concentration distribution of the hydrogen atoms in the depthdirection in the vicinity of the interface between oxide film 8 andp-type layer 4 serving as the semiconductor layer was measured as toeach of the samples described above by using a method similar to themethod of measurement in the test of Example 1 that has been alreadydiscussed. In other words, the SIMS (secondary ion mass spectroscopy)was used as a method of measurement. In addition, the channel mobilitywas measured in the formed semiconductor devices. A method similar tothe method of measurement in the test of Example 1 was used as a methodof measurement.

(Result of Measurement)

The concentration distribution of the hydrogen atoms in the depthdirection was basically similar to the concentration distribution of thenitrogen atoms shown in FIG. 18. In other words, similarly to theconcentration distribution of the nitrogen atoms shown in FIG. 18, thehydrogen atoms had the highest concentration in the interface portionbetween oxide film 8 and p-type layer 4 serving as the semiconductorlayer, and the value thereof was greater than or equal to 1×10²¹ cm⁻³.The hydrogen atoms were distributed within a range of ±10 nm around theinterface between oxide film 8 and p-type layer 4. It is noted that bothsamples for Examples 1 and 2 described above showed the substantiallysimilar concentration distribution of the hydrogen atoms. In the samplefor Example 2, however, the maximum value (peak value) of theconcentration of the hydrogen atoms was higher than that of the samplefor Example 1.

Next, as for the result of the measurement of the mobility in thechannel, the relationship similar to the previously-describedrelationship between the peak value of the concentration of the nitrogenatoms and the channel mobility as shown in FIG. 19 was obtained.

In other words, as is the case with FIG. 19, the sample for thecomparative example had the lowest peak concentration of the hydrogenatoms, and at the same time, had the lowest value of the channelmobility. On the other hand, the samples for Examples 1 and 2 both hadhigher peak concentration of the hydrogen atoms, and at the same time,had larger values of the channel mobility than that of the sample forthe comparative example. Comparing the sample for Example 1 and thesample for Example 2, the sample for Example 2 having higher peakconcentration of the hydrogen atoms had a larger value of the channelmobility than that of the sample for Example 1. It is noted that, whenan approximate line approximating the data that indicates therelationship between the peak concentration of the hydrogen atoms andthe channel mobility was plotted in the graph, a curve substantiallysimilar to the approximate line (curve) in the graph shown in FIG. 19was obtained.

Here, as compared with the conventional MOSFET using silicon, a minimumvalue required for the channel mobility to achieve lower on-stateresistance is considered as 50 cm²/Vs. Therefore, even in considerationof variations in the process, it is considered that a sufficient valueof the channel mobility can be achieved if the peak concentration of thehydrogen atoms is set to greater than or equal to 1×10²¹ cm⁻³, as is thecase with the peak concentration of the nitrogen atoms.

Example 4

Next, the details of an experiment in which water vapor was used as theatmosphere gas for heat treatment to introduce hydrogen atoms into aregion within 10 nm of an interface between a semiconductor layer and aninsulating film.

(As to Sample)

A semiconductor device having a structure shown in FIG. 1 was fabricatedas a sample. A method for producing the sample is basically similar tothe above method for producing the sample in Example 3. In other words,an epitaxial layer 3 having a thickness of 10 μm was formed on an n-typesilicon carbide substrate 2 having a thickness of 400 μm, and a p-typelayer 4 having a thickness of 1 μm was formed on epitaxial layer 3.Then, phosphorus (P) was implanted as n-type conductive impurities forn⁺ regions 5 and 6, when a value of 1×10²⁰ cm⁻³ was used as theconcentration of the impurities. In addition, the gate length (channellength L_(g)) that refers to the distance between n⁺ regions 5 and 6 wasset to 100 μm. Moreover, the gate width (channel width) was set to 200μm.

A sample that was subjected to water vapor-containing oxygen gasannealing after formation of an oxide film by a dry oxidation processwas fabricated as a sample for Example 1 of the present invention. Inaddition, a sample that was subjected to the water vapor-containingoxygen gas annealing after formation of an oxide film, and furthersubjected to an annealing process where the argon gas serving as theinert gas was used as the atmosphere (argon annealing process) wasfabricated as a sample for Example 2 of the present invention. Here, asa condition for the dry oxidation process to form an oxide film 8, acondition that the heating temperature was 1200° C. and the heating timewas 30 minutes was used. In the water vapor-containing oxygen gasannealing step for the sample for Example 1 of the present invention,the oxygen gas containing water vapor was used as the atmosphere gas,and the heating temperature was set to 1100° C. and the heating time wasset to 60 minutes. As a condition for the water vapor-containing oxygengas annealing for the sample for Example 2 of the present invention, theoxygen gas containing water vapor was used as the atmosphere gas, and acondition that the heating temperature was 1100° C. and the heating timewas 120 minutes was used. In the argon annealing process for the samplefor Example 2, the argon gas was used as the atmosphere gas, and theheating temperature was set to 1100° C. and the heating time was set to60 minutes.

In addition, a sample that was not subjected to the watervapor-containing oxygen gas annealing step after formation of a gateinsulating film was fabricated as a sample for a comparative example. Itis noted that the above oxide film in Example 1 had a thickness of 40nm, the oxide film in Example 2 had a thickness of 44 nm, and the oxidefilm in the comparative example had a thickness of 33 nm.

After the insulating film as described above was formed, a sourceelectrode 11 and a drain electrode 12 were formed, and further, a gateelectrode 10 was formed on oxide film 8 serving as the gate insulatingfilm as shown in FIG. 1, similarly to the manufacturing method in thefirst embodiment of the present invention. Source electrode 11 and drainelectrode 12 were made of nickel (Ni) and had a thickness of 0.1 μm. Inaddition, gate electrode 10 was made of aluminum (Al) and had athickness of 1 μm.

(Method of Measurement)

The concentration distribution of hydrogen atoms in the depth directionin the vicinity of the interface between oxide film 8 and p-type layer 4serving as the semiconductor layer was measured as to each of thesamples described above by using a method similar to the method ofmeasurement in the test of Example 1 that has been already discussed. Inother words, the SIMS (secondary ion mass spectroscopy) was used as amethod of measurement. In addition, the channel mobility was measured inthe formed semiconductor devices. A method similar to the method ofmeasurement in the test of Example 1 was used as a method ofmeasurement.

(Result of Measurement)

The concentration distribution of the hydrogen atoms in the depthdirection was basically similar to the concentration distribution of thenitrogen atoms shown in FIG. 18, as is the case with the test of Example3. In other words, similarly to the concentration distribution of thenitrogen atoms shown in FIG. 18, the hydrogen atoms had the highestconcentration in the interface portion between oxide film 8 and p-typelayer 4 serving as the semiconductor layer, and the value thereof wasgreater than or equal to 1×10²¹ cm⁻³. The hydrogen atoms weredistributed within a range of ±10 nm around the interface between oxidefilm 8 and p-type layer 4. It is noted that both samples for Examples 1and 2 described above showed the substantially similar concentrationdistribution of the hydrogen atoms. In the sample for Example 2,however, the maximum value (peak value) of the concentration of thehydrogen atoms was higher than that of the sample for Example 1.

Next, as for the result of the measurement of the mobility in thechannel, the relationship similar to the previously-describedrelationship between the peak value of the concentration of the nitrogenatoms and the channel mobility as shown in FIG. 19 was obtained.

In other words, as is the case with FIG. 19, the sample for thecomparative example had the lowest peak concentration of the hydrogenatoms, and at the same time, had the lowest value of the channelmobility. On the other hand, the samples for Examples 1 and 2 both hadhigher peak concentration of the hydrogen atoms, and at the same time,had larger values of the channel mobility than that of the sample forthe comparative example. Comparing the sample for Example 1 and thesample for Example 2, the sample for Example 2 having higher peakconcentration of the hydrogen atoms had a larger value of the channelmobility than that of the sample for Example 1. It is noted that, whenan approximate line approximating the data that indicates therelationship between the peak concentration of the hydrogen atoms andthe channel mobility was plotted in the graph, a curve substantiallysimilar to the approximate line (curve) in the graph shown in FIG. 19was obtained.

Here, as compared with the conventional MOSFET using silicon, a minimumvalue required for the channel mobility to achieve lower on-stateresistance is considered as 50 cm²/Vs. Therefore, even in considerationof variations in the process, it is considered that a sufficient valueof the channel mobility can be achieved if the peak concentration of thehydrogen atoms is set to greater than or equal to 1×10²¹ cm⁻³, as is thecase with the peak concentration of the nitrogen atoms.

Example 5

Next, the details of an experiment in which the gas containing nitrogenatoms and hydrogen atoms is used as the atmosphere gas for heattreatment to introduce the nitrogen atoms and the hydrogen atoms into aregion within 10 nm of an interface between a semiconductor layer and aninsulating film.

(As to Sample)

A semiconductor device having a structure shown in FIG. 1 was fabricatedas a sample. A method for producing the sample is basically similar tothe above method for producing the sample in Example 3. In other words,an epitaxial layer 3 having a thickness of 10 μm was formed on an n-typesilicon carbide substrate 2 having a thickness of 400 μm, and a p-typelayer 4 having a thickness of 1 μm was formed on epitaxial layer 3.Then, phosphorus (P) was implanted as n-type conductive impurities forn⁺ regions 5 and 6, when a value of 1×10²⁰ cm⁻³ was used as theconcentration of the impurities. In addition, the gate length (channellength L_(g)) that refers to the distance between n⁺ regions 5 and 6 wasset to 100 μm. Moreover, the gate width (channel width) was set to 200μm.

A sample that was subjected to nitrogen annealing after formation of anoxide film by a dry oxidation process was fabricated as a sample for areference example of the present invention. In addition, a sample thatwas subjected to the nitrogen annealing after formation of an oxidefilm, and further subjected to hydrogen annealing was fabricated as asample for Example 1 of the present invention. Moreover, a sample thatwas subjected to the nitrogen annealing under the condition differentfrom that of the above sample for the reference example after formationof an oxide film was fabricated as a sample for Example 2 of the presentinvention. Furthermore, a sample that was subjected to the nitrogenannealing under the condition different from that of above Example 1after formation of an oxide film, and further subjected to the hydrogenannealing was fabricated as a sample for Example 3 of the presentinvention. Here, as a condition for the dry oxidation process to form anoxide film 8, a condition that the heating temperature was 1200° C. andthe heating time was 30 minutes was used. In the nitrogen annealing stepfor the sample for the reference example of the present invention, thenitric oxide (NO) gas was used as the atmosphere gas, and the heatingtemperature was set to 1100° C. and the heating time was set to 20minutes. As a condition for the nitrogen annealing step for the samplefor Example 1 of the present invention, the nitric oxide gas was used asthe atmosphere gas, and a condition that the heating temperature was1100° C. and the heating time was 20 minutes was used. In the hydrogenannealing process for the sample for Example 1, the hydrogen gas wasused as the atmosphere gas, and the heating temperature was set to 1100°C. and the heating time was set to 30 minutes. In the nitrogen annealingstep for the sample for Example 2, the nitric oxide (NO) gas was used asthe atmosphere gas, and the heating temperature was set to 1100° C. andthe heating time was set to 60 minutes. As a condition for the nitrogenannealing step for the sample for Example 3 of the present invention,the nitric oxide gas was used as the atmosphere gas, and a conditionthat the heating temperature was 1100° C. and the heating time was 60minutes was used. In the hydrogen annealing process for the sample forExample 3, the hydrogen gas was used as the atmosphere gas, and theheating temperature was set to 1100° C. and the heating time was set to30 minutes.

In addition, a sample that was not subjected to the nitrogen annealingstep and the hydrogen annealing step after formation of a gateinsulating film was fabricated as a sample for a comparative example. Itis noted that the above oxide film in the reference example had athickness of 41 nm, the oxide film in Example 1 had a thickness of 45nm, the oxide film in Example 2 had a thickness of 41 nm, the oxide filmin Example 3 had a thickness of 45 nm, and the oxide film in thecomparative example had a thickness of 33 nm.

After the insulating film as described above was formed, a sourceelectrode 11 and a drain electrode 12 were formed, and further, a gateelectrode 10 was formed on oxide film 8 serving as the gate insulatingfilm in each of the samples as shown in FIG. 1, similarly to themanufacturing method in the first embodiment of the present invention.Source electrode 11 and drain electrode 12 were made of nickel (Ni) andhad a thickness of 0.1 μm. In addition, gate electrode 10 was made ofaluminum (Al) and had a thickness of 1 μm.

(Method of Measurement)

The total concentration distribution of the nitrogen atoms and thehydrogen atoms in the depth direction in the vicinity of the interfacebetween oxide film 8 and p-type layer 4 serving as the semiconductorlayer was measured as to each of the samples described above by using amethod similar to the method of measurement in the test of Example 1that has been already discussed. In other words, the SIMS (secondary ionmass spectroscopy) was used as a method of measurement. In addition, thechannel mobility was measured in the formed semiconductor devices. Amethod similar to the method of measurement in the test of Example 1 wasused as a method of measurement.

(Result of Measurement)

The total concentration distribution of the nitrogen atoms and thehydrogen atoms in the depth direction was basically similar to theconcentration distribution of the nitrogen atoms shown in FIG. 18. Inother words, similarly to the concentration distribution of the nitrogenatoms shown in FIG. 18, the nitrogen atoms and the hydrogen atoms hadthe highest total concentration in the interface portion between oxidefilm 8 and p-type layer 4 serving as the semiconductor layer. Thenitrogen atoms and the hydrogen atoms were distributed within a range of±10 nm around the interface between oxide film 8 and p-type layer 4.

It is noted that the peak value (maximum value) of the concentration ofthe nitrogen atoms in the above sample for the reference example was7×10²⁰ cm⁻³. In addition, the peak value of the concentration of thenitrogen atoms in the sample for Example 1 was 7×10²⁰ cm⁻³, and the peakvalue (maximum value) of the concentration of the hydrogen atoms was7×10²° cm⁻³. Moreover, the peak position of the concentration of thenitrogen atoms and the peak position of the concentration of thehydrogen atoms overlapped each other. In other words, the peak value ofthe total concentration of the nitrogen atoms and the hydrogen atoms inthe sample for Example 1 was 1.4×10²¹ cm³.

In addition, the peak value (maximum value) of the concentration of thenitrogen atoms in the above sample for Example 2 was 2×10²¹ cm⁻³.Moreover, the peak value of the concentration of the nitrogen atoms inthe sample for Example 3 was 2×10²¹ cm⁻³, and the peak value (maximumvalue) of the concentration of the hydrogen atoms was 1×10²¹ cm⁻³.Furthermore, the peak position of the concentration of the nitrogenatoms and the peak position of the concentration of the hydrogen atomsoverlapped each other. In other words, the peak value of the totalconcentration of the nitrogen atoms and the hydrogen atoms in the samplefor Example 3 was 3×10²¹ cm⁻³.

Next, as for the result of the measurement of the mobility in thechannel, the relationship similar to the previously-describedrelationship between the peak value of the concentration of the nitrogenatoms and the channel mobility as shown in FIG. 19 was obtained. Theresult of the measurement of the mobility in the channel is shown inFIG. 21. The horizontal axis in FIG. 21 indicates the peak value of thetotal concentration (peak concentration) of the nitrogen atoms and thehydrogen atoms that was measured as to each sample. It is expressed inthe unit cm⁻³. The vertical axis in FIG. 21 indicates the channelmobility in the measured semiconductor devices (MOS channel mobility).It is expressed in the unit cm²/Vs.

As shown in FIG. 21, the sample for the comparative example had thelowest peak concentration of the nitrogen atoms, and at the same time,had the lowest value of the channel mobility. On the other hand, all ofthe samples for Examples 1 to 3 had larger peak values of the totalconcentration of the nitrogen atoms and the hydrogen atoms, and at thesame time, had larger values of the channel mobility than that of thesample for the comparative example. Comparing the samples for Examples 1to 3, the sample having a larger peak value of the total concentration(peak concentration) of the nitrogen atoms and the hydrogen atoms had alarger value of the channel mobility.

Here, as compared with the conventional MOSFET using silicon, a minimumvalue required for the channel mobility to achieve lower on-stateresistance is considered as 50 cm²/Vs as already discussed. Therefore,even in consideration of variations in the process, it is consideredthat a sufficient value of the channel mobility can be achieved if thepeak value of the total concentration (peak concentration) of thenitrogen atoms and the hydrogen atoms is set to greater than or equal to1×10²¹ cm⁻³, as is the case with the peak concentration of the nitrogenatoms.

Example 6

In order to check the effect of the present invention, a prototype of asemiconductor device was fabricated and the interface state of aninterface between a semiconductor layer and an insulating film in thesemiconductor device was evaluated.

(As to Sample)

A semiconductor device shown in FIG. 22 is an MOS capacitor including asubstrate 2 that is an n-type silicon carbide substrate, a buffer layer21 formed on substrate 2, a voltage maintained layer 22 formed on bufferlayer 21, an oxide film 26 formed on voltage maintained layer 22, a gateelectrode 10 formed on the oxide film, and a rear surface electrode 31formed on a rear surface (rear surface opposite to a surface wherebuffer layer 21 is formed) of substrate 2.

The above semiconductor device was manufactured by performing thefollowing steps. Buffer layer 21 made of an n-type silicon carbideepitaxial layer was formed on the surface of substrate 2 made of n-typesilicon carbide and having a thickness of 400 μm. Substrate 2 had aspecific resistance of 0.02 Ω·cm. Buffer layer 21 had a thickness of 0.5μm and the concentration of n-type impurities was set to 5×10¹⁷ cm⁻³.Then, an n-type silicon carbide epitaxial layer 3 having a thickness of10 μm was formed on buffer layer 21 to act as voltage maintained layer22. The concentration of n-type impurities in voltage maintained layer22 was set to 5×10¹⁵ cm⁻³.

A sample that was subjected to nitrogen annealing after oxide film 26was formed on a surface of voltage maintained layer 22 by a dryoxidation process was fabricated as a sample for an example of thepresent invention. Here, as a condition for the dry oxidation process toform oxide film 26, a condition that the heating temperature was 1200°C. and the heating time was 30 minutes was used. In the nitrogenannealing step for the sample for the example of the present invention,the nitric oxide (NO) gas was used as the atmosphere gas, and theheating temperature was set to 1100° C. and the heating time was set to60 minutes.

In addition, a sample that was not subjected to the nitrogen annealingstep after formation of oxide film 26 was fabricated as a sample for acomparative example. It is noted that above oxide film 26 in the examplehad a thickness of 40 nm, and oxide film 26 in the comparative examplehad a thickness of 33 nm.

After oxide film 26 serving as the insulating film was formed asdescribed above, rear surface electrode 31 serving as an ohmic electrodewas formed on the rear surface of substrate 2, and gate electrode 10 wasformed on oxide film 26 serving as a gate insulating film as shown inFIG. 22. Rear surface electrode 31 was made of nickel (Ni) and had athickness of 0.1 μm. In addition, rear surface electrode 31 wassubjected to an alloying process (heat treatment) in the argon (Ar)atmosphere under the condition that the heating temperature was 950° C.and the heating time was 2 minutes. Gate electrode 10 was made ofaluminum (Al) and had a thickness of 1 μm. In addition, gate electrode10 had a circular two-dimensional shape whose diameter was 800 μm. Byperforming these steps, the samples for the example and the comparativeexample having the configuration of the semiconductor device shown inFIG. 22 can be obtained.

(Method of Measurement)

The capacitance-voltage characteristic (CV characteristic) was measuredas to the above samples for the example and the comparative examplehaving the configuration of the semiconductor device shown in FIG. 22(MOS capacitor). It is noted that the measurement frequency was set to 1MHz in the high-frequency CV measurement. In addition, the low-frequencyCV measurement was carried out by using a Quasistatic CV measurementmethod. It is noted that a capacitance Cs caused by a depletion layerformed on the semiconductor side of the MOS interface was determined bysolving the Poisson's equation. At this time, the inverted state was nottaken into consideration and the deep depletion state was assumed.

In addition, the interface state density was calculated as to the abovesamples for the example and the comparative example by using a High-Lowmethod. A method for calculating the interface state density by usingthe High-Low method will be outlined hereinafter.

In the above high-frequency CV measurement, a capacitance C_(it) causedby the interface state having a relatively large emission time constantdoes not appear as a capacitive component. On the other hand, in themeasurement of the CV obtained at frequencies low enough forcapture/emission of electrons from/to the interface state to respond(low-frequency CV measurement), the capacitance is observed as a valueobtained by adding the capacitance caused by the interface state to thecapacitance in the high-frequency CV measurement. Here, the capacitanceobtained by the low-frequency CV measurement includes information of theoxide film capacitance, the depletion layer capacitance and theinterface state capacitance. Therefore, a capacitance C_(LF) obtained bythe low-frequency CV measurement is expressed by the followingmathematical formula (1).

$\begin{matrix}{\left\lbrack {{Num}\mspace{14mu} 1} \right\rbrack \mspace{661mu}} & \; \\{\frac{1}{C_{LF}} = {\frac{1}{C_{ox}} + \frac{1}{C_{D} + C_{it}}}} & (1)\end{matrix}$

In the high-frequency CV measurement, however, the interface statecapacitance does not respond (is not detected) as described above, andtherefore, a capacitance C_(HF) obtained by the high-frequency CVmeasurement is expressed by the following mathematical formula (2).

$\begin{matrix}{\left\lbrack {{Num}\mspace{14mu} 2} \right\rbrack \mspace{661mu}} & \; \\{\frac{1}{C_{HF}} = {\frac{1}{C_{ox}} + \frac{1}{C_{D}}}} & (2)\end{matrix}$

Therefore, based on the above mathematical formulas (1) and (2), aninterface state density D_(it) can be determined by using the followingmathematical formula (3).

$\begin{matrix}{\left\lbrack {{Num}\mspace{14mu} 3} \right\rbrack \mspace{661mu}} & \; \\{D_{it} = {\frac{C_{ox}}{q}\left( {\frac{\frac{C_{LF}}{C_{ox}}}{1 - \frac{C_{LF}}{C_{ox}}} - \frac{\frac{C_{HF}}{C_{ox}}}{1 - \frac{C_{HF}}{C_{ox}}}} \right)\mspace{31mu} \left( {{\because C_{it}} = {qD}_{it}} \right)}} & (3)\end{matrix}$

(Result of Measurement)

The result of the above measurement will be described with reference toFIGS. 23 to 25.

In the graphs shown in FIGS. 23 and 24, the horizontal axis indicatesthe voltage and the vertical axis indicates the capacitance. Thevertical axis, however, represents the capacitance obtained bystandardizing an overall capacitance C with an oxide film capacitanceC_(ox). As can be seen from FIGS. 23 and 24, a large difference betweenthe high-frequency CV characteristic and the low-frequency CVcharacteristic was not found in the sample for the example of thepresent invention shown in FIG. 23. On the other hand, a relativelylarge difference between the high-frequency CV characteristic and thelow-frequency CV characteristic was seen in the sample for thecomparative example shown in FIG. 24. It is considered that this isbecause the capacitance caused by the interface state (interface statecapacitance) had a greater effect on the sample for the comparativeexample than on the sample for the example.

The result of the calculation of the interface state density as to thesamples for the example and the comparative example by using the aboveHigh-Low method is shown in FIG. 25. In FIG. 25, the vertical axisindicates the interface state density, and the horizontal axis indicatesthe value of the energy with respect to that of a conduction band.

As can be seen from FIG. 25, the interface state density of the samplefor the example (with the nitrogen annealing) was lower than that of thesample for the comparative example (without the nitrogen annealing) inany energy level. In addition, the interface state density of the samplefor the example was also smaller than 1×10¹² cm⁻² eV⁻¹ in an energylevel lower than 0.1 eV that of the conduction band.

Example 7

In order to check the effect of the present invention, a sample wasproduced and the relationship between the interface state density andthe MOS channel mobility was evaluated.

(As to Sample)

A semiconductor device having a structure shown in FIG. 1 was fabricatedas a sample as follows. An epitaxial layer 3 having a thickness of 10 μmwas formed on an n-type silicon carbide substrate 2 having a thicknessof 400 μm, and a p-type layer 4 having a thickness of 1 μm was formed onepitaxial layer 3. Then, phosphorus (P) was implanted as n-typeconductive impurities for n⁺ regions 5 and 6, when a value of 1×10²⁰cm⁻³ was used as the concentration of the impurities. In addition, thegate length (channel length L_(g)) that refers to the distance betweenn⁺ regions 5 and 6 was set to 100 μm. Moreover, the gate width (channelwidth) was set to 200 μm.

A sample that was subjected to nitrogen annealing after formation of anoxide film by a dry oxidation process was fabricated as a sample forExample 1 of the present invention. In addition, a sample that wassubjected to the nitrogen annealing after formation of an oxide film,and further subjected to an annealing process where the argon gasserving as the inert gas was used as the atmosphere (argon annealingprocess) was fabricated as a sample for Example 2 of the presentinvention. Here, as a condition for the dry oxidation process to form anoxide film 8, a condition that the heating temperature was 1200° C. andthe heating time was 30 minutes was used. In the nitrogen annealing stepfor the sample for Example 1 of the present invention, the NO gas wasused as the atmosphere gas, and the heating temperature was set to 1100°C. and the heating time was set to 60 minutes. As a condition for thenitrogen annealing step for the sample for Example 2 of the presentinvention, the NO gas was used as the atmosphere gas, and a conditionthat the heating temperature was 1100° C. and the heating time was 120minutes was used. In the argon annealing process for the sample forExample 2, the argon gas was used as the atmosphere gas, and the heatingtemperature was set to 1100° C. and the heating time was set to 60minutes.

In addition, a sample that was not subjected to the hydrogen annealingstep after formation of a gate insulating film was fabricated as asample for a comparative example. It is noted that the above oxide filmin Example 1 had a thickness of 40 nm, the oxide film in Example 2 had athickness of 46 nm, and the oxide film in the comparative example had athickness of 33 nm.

After the insulating film as described above was formed, a sourceelectrode 11 and a drain electrode 12 were formed, and further, a gateelectrode 10 was formed on oxide film 8 serving as the gate insulatingfilm as shown in FIG. 1, similarly to the manufacturing method in thefirst embodiment of the present invention. Source electrode 11 and drainelectrode 12 were made of nickel (Ni) and had a thickness of 0.1 μm. Inaddition, gate electrode 10 was made of aluminum (Al) and had athickness of 1 μm.

(Method of Measurement)

The channel mobility was measured in the samples of the formedsemiconductor devices. As a method of measurement, a method similar tothe method of measurement in the test of Example 1 was used.

In addition, the interface state density was calculated as to eachsample by using a method similar to the method in the test of aboveExample 6 (that is, by using the High-Low method based on the data ofthe high-frequency CV characteristic and the low-frequency CVcharacteristic).

(Result of Measurement)

The result of the measurement is shown in FIG. 26. The horizontal axisin FIG. 26 indicates the value of the interface state density in anenergy level lower than 0.1 eV that of a conduction band. It isexpressed in the unit cm⁻² eV⁻¹. The vertical axis in FIG. 26 indicatesthe channel mobility in the measured semiconductor devices (MOS channelmobility). It is expressed in the unit cm²/Vs.

As can be seen from FIG. 26, as the interface state density was lowered,the channel mobility was increased. Here, as compared with theconventional MOSFET using silicon, a minimum value required for thechannel mobility to achieve lower on-state resistance is considered as50 cm²/Vs as already discussed. It can also be seen from FIG. 26 that arange of the interface state density where the channel mobility is 50cm²/Vs is less than or equal to 7×10¹¹ cm⁻² eV⁻¹. Generally, however, alarge error may be included in the measured value of the interface statedensity. Therefore, according to the inventor's experience, it isconsidered that the sufficient channel mobility can be achieved if theinterface state density (in the energy level lower than 0.1 eV that ofthe conduction band) is set to smaller than 1×10¹² cm⁻² eV⁻¹.

It should be understood that the embodiments and the examples disclosedherein are illustrative and not limitative in any respect. The scope ofthe present invention is defined by the terms of the claims, rather thanthe description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The present invention is advantageously applied to a silicon carbidesemiconductor device where an insulating film is formed to contact witha semiconductor layer made of silicon carbide, such as an MOSFET and aDiMOSFET.

1.-30. (canceled)
 31. A silicon carbide semiconductor device,comprising: a substrate made of silicon carbide and having an off-angleof greater than or equal to 50° and less than or equal to 65° withrespect to a surface orientation of {0001}; a semiconductor layer formedon said substrate and made of silicon carbide; and an insulating filmformed to contact with a surface of said semiconductor layer, a maximumvalue of a concentration of hydrogen atoms in a region within 10 nm ofan interface between said semiconductor layer and said insulating filmbeing greater than or equal to 1×10²¹ cm⁻³.
 32. The silicon carbidesemiconductor device according to claim 31, wherein nitrogen atoms arecontained in said region within 10 nm of the interface between saidsemiconductor layer and said insulating film.
 33. The silicon carbidesemiconductor device according to claim 31, wherein an interface statedensity in an energy level lower than 0.1 eV that of a conduction bandis smaller than 1×10¹² cm⁻² eV⁻¹.
 34. The silicon carbide semiconductordevice according to claim 31, wherein said substrate has anoff-orientation in a range of less than or equal to ±5° in a <11-20>direction.
 35. The silicon carbide semiconductor device according toclaim 31, wherein said substrate has an off-orientation in a range ofless than or equal to ±5° in a <01-10> direction.
 36. The siliconcarbide semiconductor device according to claim 35, wherein a mainsurface of said substrate has a surface orientation whose off-angle isgreater than or equal to −3° and less than or equal to +5° with respectto a surface orientation of {03-38}.
 37. A method for manufacturing asilicon carbide semiconductor device, comprising the steps of: preparinga substrate made of silicon carbide and having an off-angle of greaterthan or equal to 50° and less than or equal to 65° with respect to asurface orientation of {0001}; forming a semiconductor layer on saidsubstrate; forming an insulating film to contact with a surface of saidsemiconductor layer; and adjusting a concentration of hydrogen atoms ina region within 10 nm of an interface between said semiconductor layerand said insulating film such that a maximum value of the concentrationof the hydrogen atoms is greater than or equal to 1×10²¹ cm⁻³.
 38. Themethod for manufacturing a silicon carbide semiconductor deviceaccording to claim 37, further comprising the step of: introducingnitrogen atoms into said region within 10 nm of the interface betweensaid semiconductor layer and said insulating film.
 39. The method formanufacturing a silicon carbide semiconductor device according to claim38, wherein said step of introducing nitrogen atoms includes a step ofusing a gas containing the nitrogen atoms as an atmosphere gas for heattreatment of said substrate where said insulating film is formed. 40.The method for manufacturing a silicon carbide semiconductor deviceaccording to claim 37, wherein said step of adjusting a concentration ofhydrogen atoms includes a step of using a gas containing the hydrogenatoms as an atmosphere gas for heat treatment of said substrate wheresaid insulating film is formed.
 41. The method for manufacturing asilicon carbide semiconductor device according to claim 40, wherein saidstep of adjusting a concentration of hydrogen atoms includes a step ofusing said gas containing the hydrogen atoms as the atmosphere gas forheat treatment, and then, using an inert gas as the atmosphere gas forheat treatment of said substrate.
 42. The method for manufacturing asilicon carbide semiconductor device according to claim 41, wherein saidgas containing the hydrogen atoms is water vapor or watervapor-containing oxygen.